Linux Security: Auditing, Hardening and Compliance

Using Ed25519 for OpenSSH keys (instead of DSA/RSA/ECDSA)

Introduction into Ed25519

OpenSSH 6.5 added support for Ed25519 as a public key type. It is using an elliptic curve signature scheme, which offers better security than ECDSA and DSA. At the same time it also has good performance. This type of keys may be used for user and host keys. With this in mind, it is great to be used together with OpenSSH. In this article we have a look at this new key type.

DSA or RSA

Many forum threads have been created regarding the choice between DSA or RSA. DSA is being limited to 1024 bits, as specified by FIPS 186-2. This is also the default length of ssh-keygen. While the length can be increased, it may not be compatible with all clients. So it is common to see RSA keys, which are often also used for signing. With Ed25519 now available, the usage for both will slowly decrease.

Configuring the server

The first thing to check is if your current OpenSSH package is up-to-date. You will need at least version 5.6 of OpenSSH.

Change SSH configuration (server)

Next step is changing the sshd_config file. Add the new host key type:

HostKey /etc/ssh/ssh_host_ed25519_key

Remove any of the other HostKey settings that are defined.

Client Configuration

After configuring the server, it is time to do the client. We have to create a new key first. Make sure that your ssh-keygen is also up-to-date, to support the new key type. Note: the tilde (~) is an alias for your home directory and expanded by your shell.

Insight: using -o

Normally you can use the -o option to save SSH private keys using the new OpenSSH format. It uses bcrypt/pbkdf2 to hash the private key, which makes it more resilient against brute-force attempts to crack the password. Only newer versions (OpenSSH 6.5+) support it though. For this key type, the -o option is implied and does not have to be provided. Also a bit size is not needed, as it is always 256 bits for this key type.

Are you already using the new key type? Or other tips for our readers? Leave a comment.